In order to meet increasing needs in the field of self-contained power supplies in fields as diverse as computers, video appliances, telephony, the space industry, the medical industry, microelectronics, stationary applications, hybrid vehicles . . . , several systems for storing energy are used today, including Li-ion, Ni-MH, Ni—Cd, and acid-lead accumulators.
These accumulators have different performances in terms of energy density and power density.
Whereas energy density corresponds to self-sufficiency of the storage system, power density indicates the capacity of the system of releasing a more or less significant amount of energy within a short time interval. This criterion is particularly important for new applications such as hybrid automobiles and power electronics.
Therefore Li-ion accumulators are increasingly used today as self-contained energy sources, in particular in portable equipment, where they progressively replace nickel-cadmium (Ni—Cd) and nickel-metal hydride (Ni-MH) accumulators.
For several years now, the sales of Li-ion accumulators widely have exceeded those of Ni-MH and Ni—Cd accumulators and they mainly relate to the fields of telephony and of portable computers for which self-sufficiency is a primordial criterion.
This development is explained by the continuous improvement in the performances of Li-ion accumulators in terms of energy density, thereby giving these accumulators mass and bulk energy densities much larger than those proposed by the Ni—Cd and Ni-MH technologies. Thus, the mass energy density is of more than 180 Wh/kg for Li-ion accumulators, versus 50 and 100 Wh/kg for Ni—Cd and Ni-MH accumulators respectively; while acid-lead accumulators with which automobiles are equipped, for example have an energy density of only 30 to 35 Wh/kg.
Like energy-intensive mobile applications requiring great self-sufficiency, power electronics now forms a field of application, with a future, for Li-ion accumulators capable of performing rapid charging/discharging cycles.
For this, the power performances of Li-ion accumulators may be strongly improved by the use of innovative materials of electrodes adapted to power applications.
In particular, Li4Ti5O12 titanium oxide is an alternative to the negative graphite electrode for this type of application. Due to a higher potential than that of carbon (1.55V vs. Li+/Li against 0.1V vs. Li+/Li for graphite), this oxide with a spinel structure allows rapid recharging of the accumulator without the risk of formation of lithium dendrites and therefore of short circuits and explosion. Other titanium oxides are also under study. This for example concerns titanium dioxide TiO2, with the anatase and bronze forms for example being favorable to insertion/extraction of the lithium.
Innovations are also in progress concerning the positive electrode. Various compounds may be used according to the desired voltage and capacity. Lithiated iron phosphate LiFePO4, with an olivine structure (170 mAh/g to 3.4V vs. Li+/Li), has been considered for several years as a positive electrode material of choice for certain new applications such as hybrid automobiles or portable cooling. At a less advanced stage, high voltage spinel oxides with great energy of the LiNi0.5Mn1.5O4 type are also under study with view to future replacement of commercial lamellar oxides. The latter are still widely used in commercial Li-ion accumulators.
In every case, improvements in the power performances are reckoned with in order to meet the new needs.
Unlike the performances of accumulators in terms of energy density, which mainly depend on the selection of the electrochemical pair used, since the nature of the materials of the positive and negative electrodes imposes the voltage and the capacity of the cell, the performances in terms of power of Li-ion accumulators are strongly related to the method for preparing the electrode materials which influences their aspect, their size and their morphology.
Improvements are further required in this respect in order to durably implant Li-ion accumulators in high power applications.
In other words, the emergence of new generations of Li-ion accumulators requires the use of more performing electrode materials.
In particular, it is necessary to improve the specific capacity of the electrode materials at higher (charging/discharging) rates.
The solution which is presently used the most for meeting this need consists of adding to the electrode active compound, carbon or another chemical agent with which electron conductivity may be improved.
Thus, in documents WO-A1-02/27823 [1] and WO-A2-2005/076390 [2], it has been demonstrated that the electron conducting compound such as carbon may be directly incorporated into the electrode active compound during the synthesis of the latter which thus gives the possibility of having a mixture of electrode active material and of electron conducting compound such as carbon, homogeneous and of good electrochemical quality.
In these documents, the carbon is formed by thermal decomposition under a controlled, inert or reducing atmosphere, from an organic source such as saccharose, cellulose or citric acid.
This organic source is added beforehand to the precursors for the synthesis of the electrode active material. The formation of the electrode active compound, material, and the decomposition of the organic carbon source generally take place during the same heat treatment step. This heat treatment is generally carried out at a temperature comprised between 600 and 800° C.
It should be noted that for some electrode active materials sensitive to the effect of heat, such a temperature is too high. For example, TiO2—B (bronze form), which is one of the structure varieties of TiO2, is gradually transformed into TiO2 of the anatase form from 500° C. and into TiO2 of the rutile form above 800° C., while the TiO2—B, as for it, is generally obtained between 300 and 400° C.
It is therefore not possible with such heat-sensitive electrode active materials to use the method for incorporating carbon described in the aforementioned documents [1] and [2].
Many other materials, the preparation of which takes place at a low temperature with a mild chemistry method, under hydrothermal conditions, have the same type of thermal limitation. Further, the presence of carbon, associated with a high synthesis temperature, notably above 500° C., entails the presence of a reducing atmosphere in the vicinity of the electrode active material.
This type of reaction set into play for preparing materials with the methods of documents [1] and [2] is therefore also incompatible with the synthesis of materials sensitive to reduction.
Further, a high temperature often has the consequence of increasing the size of the particles, caused by an agglomeration phenomenon, unfavorable to obtaining materials dedicated to high power applications.
In addition to the methods described in documents [1] and [2], in which the thermal decomposition of an organic source is carried out in situ simultaneously with the synthesis of the active material, a method in which carbon is used during the synthesis for reducing the degree of oxidation of the transition elements, is described in the document of J. Barker, M. Y. Saidi, J. L. Swoyer, J. Electrochem. Soc., 150 (6) (2003) A684-A688 [3]. More specifically, this document describes a carbothermal reduction (CTR) process in which carbon with high specific surface area is intimately mixed with precursor compounds of the compounds γ-LiV2O5 and Li3V2(PO4)3 and the mixture is heated in an inert atmosphere. The method of this document substantially has the same drawbacks as the methods described in documents [1] and [2].
Therefore considering the foregoing, there exists a need for a method for preparing an electrochemically active electrode material, and notably an electrochemically active electrode material comprising a mixture of an electrode active compound and of an electron conducting compound which may be applied with any kinds of electrode active compounds and of electron conducting compounds and in particular even with electrode active compounds which are heat-sensitive and/or sensitive to reduction for example by carbon.
Further there exists a need for such a method which does not cause any structural modification of the active compound, or any degradation of the latter.
There still exists a need for such a preparation method which allows preparation of an electrochemically active electrode material which has improved electrochemical performances, in particular for high (charging/discharging) rates and high powers, notably as compared with similar electrochemically active materials presently used such as those prepared in documents [1], [2] and [3].
Finally there exists a need for such a method which is simple, reliable, easy to apply and which includes a limited number of steps.
The goal of the present invention is to provide a method for preparing a mixture of an electrode active compound and of an electron conducting compound which meets the whole of the needs listed above.
The goal of the present invention is still to provide such a method which does not have the drawbacks, limitations, defects and disadvantages of the methods of the prior art and which solves the problems of the methods of the prior art.